Diversity receiver

ABSTRACT

A diversity receiver includes N number of Fourier transform circuits, N number of channel state estimators, N number of channel equalizers, N number of soft demappers, N number of noise power estimators, N number of multipliers, a combination/selection unit, and a channel decoder. The noise power estimators and multipliers provided in the diversity receiver generate individual channel weights for each channel, which serve as background noise information for the channel decoder.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The invention relates to a diversity receiver, and particularly to adiversity receiver for an orthogonal frequency division multiplexing(OFDM) system.

(b) Description of the Related Art

In a time-variant channel, interference between different sub-carriersand rapid channel fading (frequency-selective fading) may seriouslydegrade system performance and cause an error floor. Hence, a typicalorthogonal frequency division multiplexing (OFDM) system often adopts adiversity reception technique to solve the aforesaid problems. Thediversity reception technique is widely used in various applications andparticularly for mobile reception. Typically, a diversity receiverincludes two antennas and their respective signal processing units forsubsequent treatments. The two antennas are separately provided forreceiving different versions of the same transmitted signal, and thesignal processing units are used to select or combine input signals fromdifferent transmission paths.

Referring to FIG. 1, a conventional diversity receiver 10 includes twobranches 11 and 12 having similar components, a combination/selectionunit 1 a, and a Viterbi decoder 1 b. The branch 11 includes a Fouriertransform circuit 111, a channel state estimator 112, a channelequalizer 113, and a soft demapper 114. Further, the branch 12 includesa Fourier transform circuit 121, a channel state estimator 122, achannel equalizer 123, and a soft demapper 124.

When a transmitter (not shown) transmits a first version input signal I₁(n,k) regarding an nth symbol and a k_(th) sub-carrier (n and k arepositive integers) to the diversity receiver 10, the Fourier transformcircuit 111 of the branch 11 receives the first version input signalI₁(n,k) via an antenna and transforms it into a first frequency-domainsignal Y₁(n,k). On the other hand, when the transmitter transmits asecond version input signal I₂(n,k) regarding an nh symbol and a k_(th)sub-carrier to the diversity receiver 10, the Fourier transform circuit121 of the branch 12 receives the second version input signal I₂(n,k)via an antenna and transforms it into a second frequency-domain signalY₂(n,k). The fourier transform circuit 111 and the fourier transformcircuit 121 separately receive the first version input signal I₁(n,k)and the second version input signal I₂(n,k). Note that the first versioninput signal I₁(n,k) and the second version input signal I₂(n,k) includethe same data but are distinguished as being transmitted via differentantenna path or at different time.

The mathematical models for the frequency-domain signals Y₁(n,k) andY₂(n,k) are given by the following equation:

Y ₁(n,k)=H ₁(n,k)S₁(n,k)+V ₁(n,k)

Y ₂(n,k)=H ₂(n,k)S₂(n,k)+V ₂(n,k)   (1.1)

where H₁(n,k) and H₂(n,k) are respective channel frequency responses ofthe first and second versions of input signals, S₁(n,k) and S₂(n,k) aretransmission data transmitted by the transmitter, and V₁(n,k) andV₂(n,k) are Additive White Gaussian Noises (AWGN). The relationshipbetween Additive White Gaussian Noises of different channels is givenby:

σ_(V) ₁ ²≠σ_(V) ₂ ²

which indicates the signal variants of the branch 11 are different tothat of the branch 12, i.e. the background noises of the branch 11 andthat of the branch 12 are different to each other. However, it should benoted the above relationship does not mean the noises V₁(n,k) andV₂(n,k) are completely unrelated.

The channel state estimator 112 fetches the first frequency-domainsignal Y₁(n,k) and evaluates the estimate value of the channel frequencyresponse H₁(n,k) according to a reference signal (such as a pilotsignal) contained in the first frequency-domain signal Y₁(n,k). Then,the estimate value of the channel frequency response H₁(n,k) is fed tothe channel equalizer 113. Similarly, the channel state estimator 122outputs the estimate value of the channel frequency response H₂(n,k) tothe channel equalizer 123. The channel equalizer 113 receives the firstfrequency-domain signal Y₁(n,k) and generates a multiplied signalM₁(n,k) according to the estimate value of the channel frequencyresponse H₁(n,k). Similarly, in the second branch 12, the channelequalizer 123 generates another multiplied signal M₂(n,k) through thesame treatments. The multiplied signals M₁(n,k) and M₂(n,k) are givenby:

M ₁(n,k)=|H ₁(n,k)|² S ₁(n,k)+H ₁·(n,k)V ₁(n,k)

M ₂(n,k)=|H ₂(n,k)|² S ₂(n,k)+H ₂·(n,k)V ₂(n,k)   (1.2)

where H₁·(n,k) and H₂·(n,k) are respective complex conjugates of H₁(n,k)and H₂(n,k).

Next, the multiplied signal M₁(n,k) is divided by |H₁(n,k)|² by means ofa divider 113 d in the channel equalizer 113 to generate a firstequalized signal E_(O1)(n,k). Similarly, a second equalized signalE_(O2)(n,k) is generated by the same division operation performed by adivider 123 d in the channel equalizer 123 of the branch 12. Thus, weobtain:

E _(O1)(n,k)=S ₁(n,k)+{(H·(n,k)V ₁(n,k))/|H ₁(n,k)|²}

E _(O2)(n,k)=S ₂(n,k)+{(H ₂·(n,k)V ₂(n,k) )/|H ₂(n,k)²}  (1.3)

Further, the values of the divisors, namely |H₁(n,k)|² and |H₂(n,k)|² ,are fed to the combination/selection unit 1 a and serve as referenceinformation for the Viterbi decoder 1 b.

Typically, the noise term in Equation 1.3, i.e. {(H₁·(n,k)V₁(n,k))/|H₁(n,k)|² } or {(H₂·(n,k)V₂(n,k )/|H₂(n,k)|² }, is sosmall as to be neglected compared to the transmission data S₁(n,k) andS₂(n,k). Hence, the first and second equalized signal E_(O1)(n,k) andE_(O2)(n,k) can be rewritten as:

E _(O1)(n,k)=S ₁(n,k)

E _(O2)(n,k)=S ₂(n,k)   (1.4)

Then, the transmission data S₁(n,k) and S₂(n,k) can be extracted afterequalization and then respectively transmitted to the soft demappers 114and 124. The soft demappers 114 and 124 perform symbol demapping on themto respectively generate demapped signals Sf₁(n,k) and Sf₂(n,k) that arefed to the combination/selection unit 1 a.

Finally, the combination/selection unit 1 a perform either combinationor selection on the demapped signals Sf₁(n,k), Sf₂(n,k) and the valuesof the divisors |H₁(n,k)|², |H₂(n,k)|² according to their responsequalities to generate a decode signal E. The decode signal E istransmitted to the Viterbi decoder 1 b to generate a decoded data O.

However, in the conventional design, since the channel weights ofdifferent branches set by their respective channel equalizers are equalto each other, the Viterbi decoder 1 b can be provided with only channelinformation but without background noise information about each channel.Therefore, the decoding performance of the Viterbi decoder 1 b isdifficult to be improved.

BRIEF SUMMARY OF THE INVENTION

Hence, an object of the invention is to provide a diversity receiver foran OFDM system having improved decoding performance where backgroundnoise information is provided as reference decoding information for achannel decoder.

According to the invention, a diversity receiver for an OFDM systemincludes N number of Fourier transform circuits, N number of channelstate estimators, N number of channel equalizers, N number of softdemappers, N number of noise power estimators, N number of multipliers,a combination/selection unit, and a channel decoder. The diversityreceiver has N number of branches (N is a positive integer) forreceiving M number of versions (M is a positive integer) of inputsignals. A P_(th) Fourier transform circuit (P is a positive integer;1≦P≦N) receives a Q_(th) version input signal (Q is a positive integer;1≦Q≦M) and generates a P_(th) frequency-domain signal comprising atleast a P_(th) transmission data. A P_(th) channel state estimatorgenerates a P_(th) estimate channel frequency response and a P_(th)estimate transmission value according to the P_(th) frequency-domainsignal. A P_(th) channel equalizer receives the P_(th) frequency-domainsignal and generates a P_(th) equalized signal and the square of theabsolute value of the P_(th) estimate channel frequency responseaccording to the P_(th) estimate channel frequency response, and theP_(th) equalized signal comprises the P_(td) transmission data. A P_(th)soft demapper receives the P_(th) equalized signal and performs symbolmapping on the P_(th) equalized signal to generate a P_(th) outputsignal. A P_(th) noise power estimator receives the P_(th)frequency-domain signal and generates a P_(th) channel weight accordingto the P_(th) estimate transmission value. A P_(th) multipliermultiplies the P_(th) output signal by the P_(th) channel weight tooutput a P_(th) multiplication. The combination/selection unit receivesN number of the multiplications and N number of the squares of theabsolute values of the estimate channel frequency responses, and then itperforms either combination or selection on the multiplications and thesquares of the absolute values of the estimate channel frequencyresponses according to their signal qualities to generate a decodesignal. The channel decoder decodes the decode signal to generatedecoded data

Through the design of the invention, the noise power estimators andmultipliers are provided in the diversity receiver to generateindividual channel weights for each channel, which serve as backgroundnoise information for the channel decoder for subsequent treatments soas to improve the decoding performance of the channel decoder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram illustrating a conventional diversityreceiver

FIG. 2 shows a block diagram illustrating a diversity receiver of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Detail descriptions of the diversity receiver for an OFDM systemaccording to the invention will be provided in the following in order tomake the invention thoroughly understood. The symbols n, k, N, M, P, Qused in the following descriptions are positive integers.

FIG. 2 shows a block diagram illustrating an embodiment of theinvention. Referring to FIG. 2, the diversity receiver 20 for an OFDMsystem has N number of branches 21˜2N for receiving M number of versionsof input signals I₁(n,k)−I_(M)(n,k), where I_(Q)(n,k)(1≦Q≦M) representsan Q_(th) version input signal regarding an n_(th) symbol and a k_(th)sub-carrier transmitted from a transmitter. The diversity receiver 20includes N number of Fourier transform circuits 111˜1N1, N number ofchannel state estimators 112˜1N2, N number of channel equalizers113˜1N3, N number of soft demappers 114˜1N4, N number of noise powerestimators 211˜2N1, N number of multipliers 212˜2N2, acombination/selection unit 2 a, and a channel decoder 2 b.

The Fourier transform circuits 111˜1N1 are respectively provided inbranches 21˜2N. The P_(th) (1≦:P≦N) Fourier transform circuit 1P1receives the Q_(th) version input signal I_(Q)(n,k) and generates aP_(th) frequency-domain signal Y_(P)(n,k), where the P_(th)frequency-domain signal Y_(P)(n,k) at least contains a P_(th)transmission data S_(P)(n,k). The channel state estimators 112˜1N2 arerespectively provided in branches 21˜2N. The P_(th) channel stateestimator 1P2 generates a P_(th) estimate channel frequency responseH_(P)(n,k) of the P_(th) channel and a P_(th) estimate transmissionvalue Ŝ_(P)(n,k) according to a P_(th) channel reference signal (such asa pilot signal) contained in the P_(th) frequency-domain signalY_(P)(n,k). The channel equalizers 113˜1N3 are respectively provided inbranches 21˜2N. The P_(th) channel equalizer 1P3 receives the P_(th)frequency-domain signal Y_(P)(n,k) and, according to the P_(th) estimatechannel frequency response H_(P)(n,k), generates a P_(th) equalizedsignal E_(OP)(n,k) and the square of the absolute value of the P_(th)estimate channel frequency response |H_(P)(n,k)|², where equalizedsignal E_(OP)(n,k) at least contains the P_(th) transmission dataS_(P)(n,k). For example, the P_(th) equalized signal E_(OP)(n,k) can bewritten as: E_(OP)(n,k) S_(P)(n,k).

Further, the P_(th) channel equalizer 1P3 generates a P_(th) multipliedsignal M_(P)(n,k) whose value equals the multiplication of the P_(th)equalized signal E_(OP)(n,k) and the square of the absolute value of theP_(th) estimate channel frequency response |H_(P)(n,k)|², and the P_(th)channel equalizer 1P3 includes a divider 1P3 d used to divide the valueof the P_(th) multiplied signal M_(P)(n,k) by a divisor of the absolutevalue of the P_(th) estimate channel frequency response |H_(P)(n,k)|².

The soft demappers 114˜1N4 are respectively provided in branches 21∞2N.The P_(th) soft demapper 1P4 receives the P_(th) equalized signalE_(OP)(n,k) and performs symbol demapping to generate a P_(th) outputsignal Sf_(P)(n,k). The noise power estimators 211˜2N1 are respectivelyprovided in branches 21˜2N. The P_(th) noise power estimator 2P1receives the P_(th) frequency-domain signal Y_(P)(n,k) and generates aP_(th) channel weight dp according to the P_(th) estimate transmissionvalue Ŝ_(P)(n,k). The multipliers 212˜2N2 are respectively provided inbranches 21˜2N. The P_(th) multiplier 2P2 multiplies the P_(th) outputsignal Sf_(P)(n,k) by the P_(th) channel weight d_(p). Thecombination/selection unit 2 a receives N number of multiplicationsd₁×Sf₁(n,k)−d_(N)×Sf_(N)(n,k) and N number of the squares of theabsolute values of the estimate channel frequency responses|H₁(n,k)|²˜|H_(N)(n,k)|² and performs either combination or selection onthese received signals according to their signal qualities to generate adecode signal En. The decode signal En is transmitted to the channeldecoder 2 b to generate decoded data Do. The channel decoder 2 b may bea Viterbi decoder or a Reed-Solomon decoder.

The operations of the diversity receiver 20 are described as thefollowing where the first and the second branches 21 and 22 are taken asexamples. Other branches are similar in operation and thus notexplaining in detail. Further, the architecture and operation principleof the diversity receiver 20 of the invention is similar to those of theconventional diversity receiver 10, except each branch of the diversityreceiver 20 is additionally provided with a noise estimator 2P1 and amultiplier 2P2 so as to provide background noise information for thechannel decoder 2 b.

Referring to FIG. 2, a first noise power estimators 211 receives a firstfrequency-domain signal Y₁(n,k) and generates a first channel weight d₁according to a first estimate transmission value Ŝ₁(n,k). A firstmultiplier 212 multiplies a first output signal Sf₁(n,k) generated fromthe soft demapper by the first channel weight d₁, and then themultiplication d₁×Sf₁(n,k) is output by the first branch 21. Similarly,the multiplication d₂×Sf₂(n,k) is output by the second branch 22. Thecombination/selection unit 2 a receives the multiplications d₁×Sf₁(n,k)and d₂×Sf₂(n,k) and the squares of the absolute values of the estimatechannel frequency responses |H₁(n,k)|² and |H₂(n,k)|², and then itperforms either combination or selection on these received signalsaccording to their signal qualities to generate a decode signal En to beprovided for the channel decoder 2 b.

For example, if the combination treatment is performed on the outputsignals d₁×Sf₁(n,k) and d₂×Sf₂(n,k), the mathematical model of thedecode signal En can be written:

En=d ₁×soft{S ₁(n,k)}+d ₂×soft{S ₂(n,k) }  (2.1)

where the first and the second channel weights d₁ and d₂ can beobtained:

d ₁ /d ₂ =E{|{tilde over (V)} ₂ (n,k)|² }/E {|{tilde over (V)}₁(n,k)|²}  (2.2)

E{|{tilde over (V)} ₁(n,k)² }=E{Y ₁(n,k)−H ₁(n,k){tilde over(S)}(n,k)|²}

E{|{tilde over (V)} ₂(n,k)² }=E{Y ₂(n,k)−H ₂(n,k){tilde over(S)}₂(n,k)|²}  (2.3)

As shown in Equation 2.2, the first and the second channel weights d₁and d₂ are in inverse proportion to the mean square deviations of theestimate background noises V₁ and V₂. Also, the estimation equation ofthe estimate background noises V₁ and V₂ are shown in Equation 2.3. Fromthe Equation 2.2 and Equation 2.3, it is seen the first channel weightd₁ is in inverse proportion to the second channel weight d₂. Certainly,for the condition of more than two branches, the channel weightsd₁˜d_(N) can be written as:

d ₁ ×Sf ₁(n,k)=d ₂ ×Sf ₂(n,k)=. . . =d_(N) ×Sf _(N)(n,k); or

{₁ :d ₂ : . . . : d _(N)}={1/E {|V ₁(n,k)²}:1/E{|V2 (n,k)|²}:. . .:1/E{|V _(N)(n,k)|²}}

Hence, it is seen an N_(th) channel weight d_(N) is in inverseproportion to an (N−1)_(th) channel weight d_(N−1). According to theinvention, the noise power estimators 211˜2N1 are provided to generateindividual channel weights d₁˜d_(N) for each channel, which serve asbackground noise information for the channel decoder 2 b for subsequenttreatments so as to improve the decoding performance of the channeldecoder 2 b.

Further, the diversity receiver of the invention may implement varioustechniques of diversity reception, such as frequency diversity, antennaspatial diversity, antenna polarization diversity, and antenna patterndiversity.

While the invention has been described by way of example and in terms ofthe preferred embodiments, it is to be understood that the invention isnot limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications and similar arrangements aswould be apparent to those skilled in the art. Therefore, the scope ofthe appended claims should be accorded the broadest interpretation so asto encompass all such modifications and similar arrangements.

1. A diversity receiver for an orthogonal frequency divisionmultiplexing (OFDM) system having N number of branches (N is a positiveinteger) for receiving M number of versions (M is a positive integer) ofinput signals, comprising: N number of Fourier transform circuitsrespectively provided in the N number of branches, wherein a P_(th)Fourier transform circuit (P is a positive integer; 1≦P≦N) receives aQ_(th) version input signal (Q is a positive integer; 1≦Q≦M) andgenerates a P_(th) frequency-domain signal; N number of channel stateestimators respectively provided in the N number of branches, wherein aP_(th) channel state estimator generates a P_(th) estimate channelfrequency response and a P_(th) estimate transmission value according tothe P_(th) frequency-domain signal; N number of channel equalizersrespectively provided in the N number of branches, wherein a P_(th)channel equalizer receives the P_(th) frequency-domain signal andgenerates a P_(th) equalized signal and the square of the absolute valueof the P_(th) estimate channel frequency response according to theP_(th) estimate channel frequency response; N number of soft demappersrespectively provided in the N number of branches, wherein a P_(th) softdemapper receives the P_(th) equalized signal and performs symbolmapping on the P_(th) equalized signal to generate a P_(th) outputsignal; N number of noise power estimators respectively provided in theN number of branches, wherein a P_(th) noise power estimator receivesthe P_(th) frequency-domain signal and generates a P_(th) channel weightaccording to the P_(th) estimate transmission value; and N number ofmultipliers respectively provided in the N number of branches, wherein aP_(th) multiplier multiplies the P_(th) output signal by the P_(th)channel weight to output a P_(th) multiplication.
 2. The diversityreceiver as claimed in claim 1, wherein the P_(th) frequency-domainsignal comprises a P_(th) transmission data, and the P_(th) equalizedsignal comprises the P_(th) transmission data.
 3. The diversity receiveras claimed in claim 1, further comprising a combination/selection unitfor receiving N number of the multiplications and the squares of theabsolute values of the estimate channel frequency responses, andperforming either combination or selection on the multiplications andthe squares of the absolute values of the estimate channel frequencyresponses according to their signal qualities to generate a decodesignal.
 4. The diversity receiver as claimed in claim 3, furthercomprising a channel decoder for decoding the decode signal to generatedecoded data.
 5. The diversity receiver as claimed in claim 4, whereinthe channel decoder is a Viterbi decoder or a Reed-Solomon decoder. 6.The diversity receiver as claimed in claim 1, wherein the P_(th) channelequalizer generates a P_(th) multiplied signal whose value equals themultiplication of the P_(th) equalized signal and the square of theabsolute value of the P_(th) estimate channel frequency response, andthe P_(th) channel equalizer further comprises a divider used to dividethe value of the P_(th) multiplied signal by a divisor of the square ofthe absolute value of the P_(th) estimate channel frequency response. 7.The diversity receiver as claimed in claim 2, wherein the P_(th)equalized signal is written asE _(OP)(n,k)=S _(P)(n,k); where E_(OP)(n,k) is the P_(th) equalizedsignal and S_(P)(n,k) is the P_(th) transmission data.
 8. The diversityreceiver as claimed in claim 1, wherein the P_(th) frequency-domainsignal comprises a P_(th) reference signal, and the P_(th) channel stateestimator estimates the P_(th) estimate transmission value according tothe P_(th) reference signal.
 9. The diversity receiver as claimed inclaim 8, wherein the P_(th) reference signal is a pilot signal.
 10. Thediversity receiver as claimed in claim 1, wherein the P_(th) channelweight is in inverse proportion to a (P−1)_(th) channel weight.
 11. Adiversity receiver for an orthogonal frequency division multiplexing(OFDM) system for receiving M number of versions (M is a positiveinteger) of input signals at different time, comprising: a Fouriertransform circuit for receiving a Q_(th) version input signal (Q is apositive integer; 1≦Q≦M) and generates a P_(th) (P is a positiveinteger; 1≦P≦M)frequency-domain signal; a channel state estimator forgenerating a P_(th) estimate channel frequency response and a P_(th)estimate transmission value according to the P_(th) frequency-domainsignal; a channel equalizer for receiving the P_(th) frequency-domainsignal and generating a P_(th) equalized signal and the square of theabsolute value of the P_(th) estimate channel frequency responseaccording to the P_(th) estimate channel frequency response; a softdemapper for receiving the P_(th) equalized signal and performing symbolmapping on the P_(th) equalized signal to generate a P_(th) outputsignal; a noise power estimator for receiving the P_(th)frequency-domain signal and generating a P_(th) channel weight accordingto the P_(th) estimate transmission value; and a multiplier formultiplying the P_(th) output signal by the P_(th) channel weight tooutput a P_(th) multiplication.
 12. The diversity receiver as claimed inclaim 11, wherein the P_(th) frequency-domain signal comprises a P_(th)transmission data, and the P_(th) equalized signal comprises the P_(th)transmission data.
 13. The diversity receiver as claimed in claim 11,further comprising a combination/selection unit for receiving M numberof the multiplications and the squares of the absolute values of theestimate channel frequency responses, and performing either combinationor selection on the multiplications and the squares of the absolutevalues of the estimate channel frequency responses according to theirsignal qualities to generate a decode signal.
 14. The diversity receiveras claimed in claim 13, further comprising a channel decoder fordecoding the decode signal to generate decoded data.
 15. The diversityreceiver as claimed in claim 14, wherein the channel decoder is aViterbi decoder or a Reed-Solomon decoder.
 16. The diversity receiver asclaimed in claim 11, wherein the channel equalizer generates a P_(th)multiplied signal whose value equals the multiplication of the P_(th)equalized signal and the square of the absolute value of the P_(th)estimate channel frequency response, and the channel equalizer furthercomprises a divider used to divide the value of the P_(th) multipliedsignal by a divisor of the square of the absolute value of the P_(th)estimate channel frequency response.
 17. The diversity receiver asclaimed in claim 12, wherein the P_(th) equalized signal is written asE _(OP)(n,k)=S _(P)(n,k); where E_(OP)(n,k) is the P_(th) equalizedsignal and S_(P)(n,k) is the Ph transmission data.
 18. The diversityreceiver as claimed in claim 11, wherein the P_(th) frequency-domainsignal comprises a P_(th) reference signal, and the channel stateestimator estimates the P_(th) estimate transmission value according tothe P_(th) reference signal.
 19. The diversity receiver as claimed inclaim 18, wherein the P_(th) reference signal is a pilot signal.
 20. Thediversity receiver as claimed in claim 11, wherein the P_(th) channelweight is in inverse proportion to a (P−1)_(th) channel weight.